The rules for delve have changed slightly since it was last in an expansion. Previously, delve reduced the cost to cast a spell. Under the current rules, you exile cards from your graveyard at the same time you pay the spell’s cost. Exiling a card this way is simply another way to pay that cost.

9/20/2014

Delve doesn’t change a spell’s mana cost or converted mana cost. For example, Dead Drop’s converted mana cost is 10 even if you exiled three cards to cast it.

9/20/2014

You can’t exile cards to pay for the colored mana requirements of a spell with delve.

9/20/2014

You can’t exile more cards than the generic mana requirement of a spell with delve. For example, you can’t exile more than nine cards from your graveyard to cast Dead Drop.

9/20/2014

Because delve isn’t an alternative cost, it can be used in conjunction with alternative costs.